In certain applications it is desired to inject radiation from a first (reference or master) laser into a second (slave) laser in order to force the second laser to emit radiation at the same, or close to the same, frequency as the first laser. To get a high peak power from the second laser, it is usually equipped with a Q-switch. Finally, it is frequently desired to induce a frequency-shift between the two lasers. Lasers of this type are used in, for example, coherent laser radar systems. A system utilizing two lasers in this manner was described by Henderson et al. in U.S. Pat. No. 5,237,331. In that system, separate devices were used for injection-seeding, Q-switching, and frequency-shifting. This approach requires many pans, which increases cost and reduces reliability.
The present invention uses one device to replace the three separate devices, permitting a simpler way of meeting all three functions of injection-seeding, Q-switching, and frequency-shifting. The invention also provides a means to inject a relatively large signal into the slave laser, which ensures that the injection-seeding process is efficient.
An acousto-optic (AO) element is used as such a multiple-function device in the present invention. Acousto-optical devices are well known in the field of photonics. When aligned properly and with radio-frequency power applied to create an acoustic traveling wave, they can be used as Q-switches. In this case, the acoustic wave acts to diffract a light beam out from a laser cavity, thereby increasing the laser cavity loss (`low-Q` state) and preventing the laser from oscillating. During the time that the rf-power is applied, the laser material is pumped to create a high population inversion without laser action. At the end of the pumping period, the rf-power is rapidly switched off. Following switch-off no diffraction takes place and hence the laser cavity loss is reduced. In this `high-Q` state, the ratio of gain to cavity loss is high, and laser radiation builds up in the cavity until the gain is saturated to equal the cavity loss. The result of this process is that a pulse of radiation is produced. This process of producing a pulsed laser output by changing the Q of the cavity is refered to as Q-switching.
Radiation diffracted from the acoustic wave is shifted in frequency by multiples of the applied rf-frequency f.sub.rf. The n.sup.th diffraction order produces a beam shifted by n.f.sub.rf. Acousto-optical devices can therefore also be used as frequency-shifters. The various diffracted orders are also separated angularly, which makes it possible to inject radiation into an acousto-optical device without spatially interfering with a beam propagating along the zero-order diffraction axis.
The present invention uses the aforementioned properties of an AO device to accomplish injection-seeding, frequency-shifting, and Q-switching.